Wireless communication systems have become pervasive and ubiquitous to the point where data rate and quality of service requirements have become comparable to those of wired communication systems. Next generation wireless systems incorporate multiple-input multiple-output (MIMO) techniques to achieve their performance goals. MIMO systems promise higher channel capacities compared to single antenna systems by exploiting the spatial characteristics of the multipath wireless propagation channel. The theoretical performance gain achievable by MIMO systems is limited due to a number of practical design factors, including the design of the antenna array and the amount of inter-array element mutual coupling. While mutual coupling can be alleviated by increasing the spacing between array elements, accommodating multiple antennas with large spacing in modern consumer devices may be impossible due to stringent space constraints. In order to meet such demanding, and often contradictory, design criteria, antenna designers have been constantly driven to seek better materials on which to build antenna systems.
As disclosed in U.S. Pat. No. 6,933,812, metamaterials are a broad class of synthetic materials that could be engineered to wield permittivity and permeability characteristics to system requirements. It has been theorized that by embedding specific structures (usually periodic structures) in some host media (usually a dielectric substrate), the resulting material can be tailored to exhibit desirable characteristics. These materials have drawn a lot of interest recently due to their promise to miniaturize antennas by a significant factor while operating at acceptable efficiencies.
Metamaterials having permeabilities and permittivities greater than that of a host dielectric have been used to design miniaturized patch antennas. For example, Buell, Mosallaei, and Sarabandi disclose such patch antennas in an article entitled “A Substrate for Small Patch Antennas Providing Tunable Miniaturization Factors,” IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 1, January 2006. Buell et al. describe a metamaterial spiral loop unit cell that achieves permeability enhancement by aligning the magnetic field along the axis perpendicular to the spiral loop unit cell. Buell et al. further teach that a two-dimensional array of resonant spirals may be formed and the resulting substrate layers stacked to form a three-dimensional effective medium that approximates an infinite magnetic medium. Permittivity enhancement only occurs for electric fields directed in the two-dimensional plane of the spiral loop unit cell. Thus, the patch antenna described by Buell exhibits orientation-dependent permeability and permittivity at orientations that may support the modes of a microstrip patch antenna.
Though such magnetic permeability enhanced metamaterial (MPEM) substrates used for antenna miniaturization do reduce the size of the antenna, they are mostly considered unsuitable for many devices due to the accompanying increase in the substrate thickness. Also, magnetic permeability enhancement in known implementations of magnetic permeability enhanced metamaterial (MPEM) substrates are uni-directional. This is due to the uni-directional alignment of the unit cells. Only magnetic fields that are oriented in a direction perpendicular to the plane of the unit cells couple energy to them. This imposes the limitation that only antenna designs that generate uni-directional magnetic fields can fully utilize the permeability enhancement provided by the substrate, thus seriously limiting the utility of the substrate.
A multi-directional substrate is desirable that provides multi-directional permeability enhancement at a given frequency in the substrate without being restricted to a unique solution. In particular, a substrate that allows the substrate designer to pick dimensions that are more suitable for the target platform is desired.